With the progress of small-size, high-speed and lightweight electronic devices such as cellular phones and personal computers, the capacitor used for these electronic devices is demanded to be smaller in size and lighter in weight and to have a larger capacitance and a lower ESR (Equivalent Series Resistance).
As an example of such a capacitor, the electrolytic capacitor has been proposed, which capacitor is produced by anodically oxidizing an anode body for capacitors comprising a sintered body made of a valve-acting metal powder which can be anodized such as tantalum to form a dielectric layer made of the oxide of the metal on the surface of the anode body.
The electrolytic capacitor using tungsten as a valve-acting metal and employing the sintered body of the tungsten powder as an anode body can attain a larger capacitance compared to the electrolytic capacitor employing an anode body of the same volume which uses tantalum powder of the same particle diameter and is treated with the same formation voltage. However, the electrolytic capacitor having the sintered body of the tungsten powder has been unpracticed as an electrolytic capacitor due to the large leakage current (LC). In order to solve this issue, a capacitor using the alloy of tungsten and other metals has been studied and has achieved some improvement in the leakage current, but it was not enough (JP-A-2004-349658; Patent Document 1).
Patent Document 2 (JP-A-2003-272959) discloses a capacitor using an electrode of a tungsten foil having formed thereon a dielectric layer selected from WO3, W2N and WN2, but the capacitor is not to solve the above-mentioned leakage current problem.
Also, Patent Document 3 (WO 2004/055843 publication (U.S. Pat. No. 7,154,743 B2)) discloses an electrolytic capacitor using an anode body selected from tantalum, niobium, titanium and tungsten, but it does not describe a specific example using tungsten in the specification.
Patent Document 4 (WO 2012/086272 publication) discloses an electrolytic capacitor using as an anode a sintered body of tungsten powder containing silicon, which capacitor can solve the leakage current (LC) problem. The document discloses a method of incorporating silicon in a tungsten powder, using a commercially-available tungsten powder or a tungsten powder reduced with hydrogen and the like as a material tungsten powder.
In an anode body for an electrolytic capacitor which is obtained by molding tungsten powder and sintering it, a smaller particle size enables the production of an anode body having a larger capacitance, if the volume of the anode body is the same. Therefore the smaller size of the material tungsten powder is more preferable, but the average particle size of a commercially-available tungsten powder is about 0.5 to 20 μm.
Tungsten powder can be manufactured by treating oxide, halide or ammonium salt of tungsten as a raw material with a reducing agent such as hydrogen. However, increase in the rate of reduction may give rise to a problem of generating a composite oxide and the like. Therefore, it is necessary to decrease the rate of reduction in order to produce finer powder, and it leads to low production efficiency and high cost. Also, it is necessary to produce the fine powder by a complicated process equipped with an expensive controlling device. Furthermore, there has been a problem of handling a material having a wide explosibility range such as hydrogen gas.